When displaying blood vessels of a patient by way of X-ray images, e.g. in the case of CT (computerized tomography) angiography, based on recording with iodine as the contrast medium iodine is administered to the patient just before the start of a scan. At the time of recording, the vessels or lumens to be examined thereby are enriched with the contrast medium. Due to its high atomic number iodine causes a strong X-ray attenuation and therefore provides a clearly visible contrast, so the state of the vessels can be assessed very easily. However, there are often difficulties when displaying the vessel if calcifications occur in the vessel to be examined.
Due to its similarly very high intrinsic contrast, the calcium is shown enlarged and outshines the visible lumen. The calcifications therefore appear much larger in the X-ray image than they actually are. Colloquially this artifact is called the “blooming effect” or “calcium blooming”. It is primarily based on the combination of the high intrinsic contrast of the calcium with a relatively soft reconstruction kernel during image generation from the raw data and a windowing of the X-ray image optimized to the iodine contrast and habitual for the observer. In contrast to a hard reconstruction kernel, a soft reconstruction kernel obscures object edges and thereby reduces the resolution but causes less noise in the reconstructed X-ray image. From a clinical perspective it would be desirable to display a calcification as far as possible in its “original size” during the course of reconstruction or during presentation in order to be able to meaningfully medically assess the blood vessel and the extent of its calcification.
One possibility for reducing the blooming in the presentation of calcifications consists in a reconstruction having very sharp reconstruction kernels which depict object edges more sharply in space. This leads to improved mapping of the calcification due to the reconstruction and therewith to a meaningful presentation of the calcification within the iodine windowing. At the same time, however, this method can only be used to a limited extent since the noise in the image increases significantly, and consequently this often can no longer be used for diagnostic purposes.
Alternatively it is known by way of suitable combination of an image with and an image without contrast medium to calculate the signal component caused by the calcium from the X-ray image. Since the calcium blooming occurs in both images, this effect is cancelled out with subtraction and only the lumen remains. However, with this process a further scan is required, and this means additional exposure to radiation for the patient and requires additional computing effort (e.g. registration) to compensate possible, movement-determined deviations in the images due to the temporal offset of the scans.
Furthermore, there are methods for the identification and elimination of the calcium on the basis of dual-energy systems, such as described, for example, in patent application DE 10 2011 004 120 A1. Therein X-ray images of a patient are generated with two different X-ray quantum energies which are then used to identify the X-ray attenuation caused by calcium and iodine and to suppress the attenuation in the images caused by calcium such that more realistic mapping of the of calcifications results. However, this procedure also requires additional exposure to radiation for the patient, because, as a rule, the X-ray images are acquired simultaneously with two X-ray sources (dual source) having different acceleration voltage or via one X-ray source and fast kV switching. Furthermore, owing to its spatial or temporal offset, an evaluation of this X-ray data is limited to observations in the image space.
By contrast, it is the object of the present invention to provide improved measures for generating X-ray image data corrected by the calcium blooming which overcome said drawbacks of the prior art.